Back to EveryPatent.com
| United States Patent |
5,073,210
|
|
Humpston
, et al.
|
December 17, 1991
|
Method of making electrical conductors
Abstract
A method of making material in the form of a fine wire or thin strip or
sheet suitable for use as electrical conductors comprises: forming an
ingot substantially consisting of an alloy of gold and one or more of the
metals titanium, lutetium, zirconium, scandium and hafnium, the amount of
the or each of said one or more of the metals in the alloy lying in the
range from substantially 0.1 at % to an upper limit comprising the atomic
percentage of that metal corresponding to the maximum solubility of that
metal in the alloy; solution heat treating the ingot and then quenching
the ingot from the solution heat treatment temperature; mechanically
working the ingot to have a maximum dimension in at least one direction of
not greater than 250 .mu.m; and heat treating the resulting material at a
temperature below its solvus temperature from an ambient atmosphere
containing oxygen and/or nitrogen in the ambient atmosphere. The
conductors exhibit high tensile strength and resistance to elongation
under load and retention of such properties after being heated to elevated
temperatures for extended periods of time, such as to render them
particularly suitable for use as bond-wires in integrated circuit
packages.
| Inventors:
|
Humpston; Giles (Herts, GB);
Jacobson; David M. (Middlesex, GB)
|
| Assignee:
|
The General Electric Company, p.l.c. (GB2)
|
| Appl. No.:
|
527050 |
| Filed:
|
May 22, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
148/522; 148/430; 148/535; 148/536; 148/557; 148/678; 257/E23.01 |
| Intern'l Class: |
C22F 001/02; C22C 005/02 |
| Field of Search: |
148/11.5 R,430
|
References Cited
U.S. Patent Documents
| 3922180 | Nov., 1975 | Fuchs et al. | 148/11.
|
| 4387073 | Jun., 1983 | Westbrook | 420/507.
|
| 4529667 | Jul., 1985 | Shiga et al. | 428/646.
|
| 4911769 | Mar., 1990 | Yamada et al. | 148/430.
|
| Foreign Patent Documents |
| 0178481 | Apr., 1986 | EP.
| |
| 0190648 | Aug., 1986 | EP.
| |
| 3716106 | Jan., 1989 | DE.
| |
| 60-85546 | May., 1985 | JP.
| |
| 60-150657 | Aug., 1985 | JP.
| |
| 1110045 | Apr., 1968 | GB.
| |
| 1149597 | Apr., 1969 | GB.
| |
| 1491155 | Nov., 1977 | GB.
| |
Other References
Haeussler et al., "Preparation and case hardening of dispersion hardened
copper alloys", Technik, vol. 28, No. 10, 1973, pp. 622-627, Chemical
Abstracts #80(14):73373k.
"Binary alloy phase diagrams Ac-Au to Fe-Rh", by T. B. Massalski appearing
in American Society for Metals, vol. 1, 1988, pp. 326, 340, 310, 266 and
277.
Article entitled "The development of 990 Gold-Titanium: its production, use
and properties", by G. Gafner in Gold Bulletin, 1989, 22(4), pp. 112-122.
Article entitled "The promise of 990 gold", by A. M. Tasker in Aurum 34,
pp. 62-67.
Article entitled "Dispersion Hardened Gold", by Poniatowski et al., in Gold
Bulletin, 1972, 5, p. 34.
|
Primary Examiner: Dean; H.
Assistant Examiner: Schumaker; David W.
Attorney, Agent or Firm: Kirschstein, Ottinger, Israel & Schiffemiller
Claims
We claim:
1. A method of making material in the form of a fine wire or thin strip or
sheet suitable for use as electrical conductors comprising the steps of
forming an ingot substantially consisting of an alloy of gold and at least
one of the metals titanium, lutetium, zirconium, scandium and hafnium, the
amount of each of said at least one of the metals in the alloy lying in
the range from substantially 0.1 at% to an upper limit comprising the
atomic percentage of that metal corresponding to the maximum solubility of
that metal in the alloy; solution heat treating the ingot and then
quenching the ingot from the solution heat treatment temperature;
mechanically working the ingot to have a maximum dimension in at least one
direction not greater than 250 .mu.m; and heat treating the resulting
material at a temperature below its solvus temperature in an ambient
atmosphere containing at least one of oxygen and nitrogen to produce a
reaction produce precipitate with at least one of oxygen and nitrogen from
the ambient atmosphere.
2. A method according to claim 1 wherein the atomic percentage of titanium
in the alloy is in the range 0.5% to 5.0%.
3. A method according to claim 1 wherein the atomic percentage of lutetium
is in the range 0.5% to 3.85%.
4. A method according to claim 1 wherein the atomic percentage of zirconium
is in the range of 0.5% to 4.0%.
5. A method according to claim 1 wherein the atomic percentage of scandium
is in the range 0.5% to 4.4%.
6. A method according to claim 1 wherein the atomic percentage of hafnium
is in the range 0.5% to 2.5%.
7. A method according to claim 1 wherein the material resulting from the
mechanical working step has a maximum dimension in at least one direction
of less than 50 .mu.m.
8. A method according to claim 1 wherein said heat treating at a
temperature below its solvus temperature is carried out at a temperature
in the range 100.degree. to 500.degree. C.
9. A method according to claim 8 wherein said heat treating at a
temperature below its solvus temperature is carried at a temperature of
substantially 350.degree. C.
10. A method according to claim 1 wherein, after the heat treating at a
temperature below its solvus temperature step, the resulting material is
provided with a coating of at least one of gold, silver, aluminium and
copper.
11. A method according to claim 10 wherein said coating has a thickness not
substantially less than the dimension of said resulting material in said
at least one direction.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of making electrical conductors.
More precisely, the invention relates to electrical conductors, and methods
of making material in the form of fine wire or thin strip or sheet,
suitable for use as electrical conductors in electronic circuits e.g. when
in the form of wires, for use as bond-wires in integrated circuit
packages.
2. Description of Related Art
Such conductors are required to exhibit high tensile strength and
resistance to elongation under load and retention of such properties after
being heated to elevated temperatures for extended periods of time. These
properties are required in order that such conductors may have good
resistance to deformation and breakage under vibration, thermal fatigue
and mechanical shock, during and after manufacture of the circuits in
which they are employed. It will be understood in this connection that the
manufacture of electronic circuits, more especially integrated circuits,
normally involves heat treatments.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of making
materials as aforesaid which exhibit superior performance in the above
mentioned respects compared with materials made using known methods.
According to the present invention there is provided a method of making
material in the form of fine wire or thin strip or sheet suitable for use
as electrical conductors comprising: forming an ingot substantially
consisting of an alloy of gold and one or more of the metals titanium,
lutetium, zirconium, scandium and hafnium, the amount of the or each of
said one or more of the metals in the alloy lying in the range from
substantially 0.1 at% to an upper limit comprising the atomic percentage
of that metal corresponding to the maximum solubility of that metal in the
alloy; solution heat treating the ingot and then quenching the ingot from
the solution heat treatment temperature; mechanically working the ingot to
have a maximum dimension in at least one direction not greater than 250
.mu.m; and heat treating the resulting material at a temperature below its
solvus temperature in an ambient atmosphere containing oxygen and/or
nitrogen to produce a reaction product precipitate with the oxygen and/or
nitrogen from the ambient atmosphere.
For ingots consisting of gold and a single said metal, the upper limits for
the various metals are: 10 atomic percent titanium; 8.8 atomic percent
scandium; 8.0 atomic percent zirconium; 7.7 atomic percent lutetium and 5
atomic percent hafnium.
In a preferred method according to the invention the material resulting
from the mechanical working step, has a maximum dimension in at least one
direction of less than 50 .mu.m.
In one particular method in accordance with the invention, after the heat
treating below its solvus temperature step, the resulting material is
provided with a coating of gold, silver, aluminium or copper or of an
alloy or combination thereof, i.e. comprising layers of two or more of the
metals.
The invention also provides an electrical conductor having a maximum
dimension in at least one direction not greater than 250 .mu.m and
consisting at least partly of a precipitation hardened alloy of gold.
In such a conductor said alloy is preferably an alloy of gold and one or
more of the metals titanium, lutetium, zirconium, scandium and hafnium.
The conductor is then suitably made by a method according to the
invention.
One particular embodiment of an electrical conductor according to the
invention comprises a central core of said precipitation hardened alloy
provided with a coating of gold, silver, aluminium or copper, or an alloy
or combination thereof.
Alloys of gold with titanium are already known for use in the jewellery and
coinage industries (see EP-A1-0190648), such alloys exhibiting lustre, hue
and wear resistance properties which are desirable in those industries.
The present invention resides in the discovery that such alloy, and the
other alloys lying in the ambit of the present invention, when
appropriately treated exhibit properties, i.e. high tensile strength and
resistance to elongation under load, and retention of such properties
after heating at elevated temperatures, which make them very suitable for
use as fine wire or thin strip or sheet electrical conductors in
electronic circuits and for other electrical applications.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention will now be further explained and various examples of methods
of making electrical conductors according to the invention described.
One significant feature of the gold alloys used in methods according to the
present invention is that they possess precipitation hardening
characteristics. Hence, in a method according to the invention the lower
limit of the range of alloying metal in the gold alloy used is defined by
the maximum solid solubility of the alloying metal in gold at room
temperature, i.e. the lower limit is the minimum atomic percentage
(substantially 0.1 at%) required to form a precipitate phase.
Similarly the upper limit of the range of alloying metal is the atomic
percentage corresponding to the maximum solubility of the alloying metal
in gold, more particularly at the temperature of fusion of the saturated
alloy, i.e. the temperature of the eutectic or peritectic reaction between
gold and the alloying metal.
In practice the useful composition range is likely to be more restricted.
Thus, the minimum useful concentration of alloying metal is determined by
the minimum addition to gold that generates a significant strengthening
effect. This generally means that the minimum level of the addition should
be approximately five times the metallurgical minimum, e.g. for Au-Ti, the
Ti concentration should be at least 0.5 at% (rather than 0.1 at%). The
same is true for the other alloying metals.
The upper practical limit is defined by the introduction of oxides and
other non-metals with the alloying metal that impair the workability of
the alloy down to a fine wire or thin strip or sheet. The upper limit of
additive concentration according to this criterion is typically a half of
the metallurgical maximum concentration e.g. for titanium, the upper
practical limit is 5.0 at% i.e. one half of the 10 at% maximum solubility
limit of titanium in gold.
The desired precipitation hardening characteristics are best obtained with
the following combination of properties:
(a) The alloying metal has a high solid solubility in gold at elevated
temperatures and a relatively low solid solubility at room temperature.
The greater is this solubility difference the larger will be the fraction
of the precipitate phase that can form on cooling. The degree of
strengthening that can be obtained by precipitation hardening will thereby
be correspondingly greater.
(b) A large proportion of the alloying metal is retained in super-saturated
solid solution on quenching from an elevated temperature solution
treatment. This ensures a large volume fraction of precipitate generated
in a subsequent heat treatment, and thereby a high strengthening effect.
(c) The precipitate phase is a stable intermetallic compound and is harder
than the gold-rich matrix phase. This compound has as high an atomic
proportion of gold as possible, so that its volume fraction can be high in
relation to the amount of alloying metal added in relation to the amount
of alloying metal added and the degree of strengthening be made all the
greater.
A second significant feature of the gold alloys used in methods according
to the present invention is that the alloying metal has a high reactivity
with the oxygen and/or nitrogen in the ambient atmosphere. The precipitate
phase can then be made to react with the ambient atmosphere to form a
reaction product that is stable against further heat treatment.
On the basis of these criteria the favourable precipitation hardening
characteristics of the alloys used in methods according to the invention
can be seen from the following Table.
TABLE 1
______________________________________
Maximum Solubility Precipitate
Alloying solubility at 20.degree. C.
compound
element at % .degree.C.
at % formed
______________________________________
Ti 10.0 1115 0.2 Au.sub.6 Ti
Sc 8.8 808 0.2 Au.sub.4 Sc
Zr 8.0 1040 0.2 Au.sub.3 Zr
Lu 7.7 890 0.2 Au.sub.4 Lu
Hf 5.0 1064 0.2 Au.sub.5 Hf
______________________________________
Of the alloys listed, gold-titanium best meets the combination of
properties (a) to (c) above. The others also provide a significant degree
of precipitation, but not to the same extent.
By appropriate thermal treatment below the solvus temperature, more
particularly between 100.degree. C. and 500.degree. C., when in a
sufficiently thin form, i.e. having a maximum dimension in at least one
direction not greater than 250 .mu.m, it is possible to boost the strength
of these precipitation hardening alloys and their resistance to elongation
under load, and/or to retain the precipitation strength at elevated
temperatures for periods much longer than is normally possible with
precipitation hardening alloys. The reason for this additional improvement
appears to be that the reactive alloying metals combine with nitrogen and
oxygen, e.g. in the atmosphere, as illustrated in Table II below, to form
finely dispersed insoluble compounds which are then stable against further
growth or dissolution.
There is a maximum dimension in a method of making a material according to
the invention for which this strength boosting mechanism is effective
because it requires reaction to occur between the reactive metal and the
ambient atmosphere throughout a substantial proportion of the material.
This maximum dimension is substantially 50 .mu.m. No minimum dimension for
this effect has been encountered. For material having a maximum dimension
in at least one direction between 50 .mu.m and 250 .mu.m it is not
possible to boost strength significantly, but only to stabilise the
strengthening due to the precipitation process against further change.
The above factors are illustrated in Table II below which shows the
strengthening produced in wires made by a method according to the
invention from an ingot consisting of Au-1wt%Ti (Au-4at%Ti) subjected to
heat treatment at 350.degree. C. after drawing down from the ingot.
TABLE II
______________________________________
Time at Tensile strength (MPa)
350.degree. C.
conductor diameter (.mu.m)
hours 25 55 200 1040
______________________________________
0 499 437 349 207
10 998 464 470 565
100 1438 491 485 470
1000 1038 474 472 366
______________________________________
It will be seen that the wire of diameter 25 .mu.m exhibits boosted, stable
strength whilst the wires of diameter 55 .mu.m exhibits 200 .mu.m exhibit
stable precipitation strength only, and the wire of diameter 1040 .mu.m
exhibits only conventional unstable precipitation strength.
Of the metals that can be used to precipitation harden gold alloy, titanium
is the least reactive towards oxygen and also towards nitrogen. As a
general rule, the reactivity of a metal with oxygen increases with the
enthalpy of formation of its oxide and Table III below therefore indicates
the relative tendency of the precipitation hardened alloys to form stable
precipitates.
TABLE III
______________________________________
Enthalpy of formation
Oxide -.DELTA.H(298) KJ
Per mole of oxygen, KJ
______________________________________
TiO.sub.2
944 944
ZrO.sub.2
1086 1086
HfO.sub.2
1113 1113
Sc.sub.2 O.sub.3
1906 1270
Lu.sub.2 O.sub.3
1906 1270
______________________________________
It is apparent from Table III that the other alloying metals are at least
as effective in regard to this property as titanium and that suitably
heat-treated fine wires, strips or sheets of gold alloyed with the other
metals also have an enhanced strength and resistance to thermal ageing.
One method of making material in accordance with the invention will now be
described by way of example.
The material is fine wire suitable for use as bond-wires in integrated
circuits.
As a first step in the method measured quantities of gold and titanium in
the proportion Au-1wt%Ti (Au-4at%Ti) are heated together in known manner
to form a molten alloy i.e. at a temperature above 1063.degree. C. The
molten alloy is then poured into a mould and cooled to form an ingot.
The ingot is then subjected to a heat treatment for at least one hour,
typically for one hour at a temperature above 950.degree. C., e.g. to form
a solid solution of titanium in gold. The ingot is then quench cooled in
water from the solution heat treatment temperature.
The ingot is then drawn-down in known manner to a fine wire, typically of
diameter 25 .mu.m.
The wire is then heat treated at a temperature of 350.degree. C. in air for
a suitable period, as further explained below, to boost the strength of
the wire, increase its resistance to elongation, and enable it to retain
these properties at elevated temperatures for long periods, by the
mechanism described above.
The tensile strength and elongation to failure of the wire after heat
treatment for various lengths of time at 350.degree. C. are given in Table
IV below, together with comparable data for pure gold wire. The initial
condition of the wire is unannealed, hard drawn.
TABLE IV
______________________________________
Tensile Elongation to
Time at Strength (grams)
Failure (%)
350.degree. C. (hrs)
Au Au-1 wt % Ti
Au Au-1 wt % Ti
______________________________________
0 12 25 2 2
0.16 2 28 10 3
1 1 30 20 3
50 1 67 40 3
100 1 72 20 4
500 1 62 10 4
1000 1 55 10 5
5000 1 61 10 5
______________________________________
Tensile strength is expressed as the load in grams required to cause the
wire to fail. Elongation to failure was tested using a 25 mm length of
wire.
It will be seen from Table IV that the Au-Ti fine wire has a tensile
strength which is initially about twice that of pure gold wire and which
actually increases after exposure to elevated temperature for prolonged
periods, rather than markedly decreasing as is the case for pure gold
wire. At the same time the elongation of the Au-Ti wire, which is
initially the same as for pure gold wire, increases only slightly on
exposure to elevated temperature, whereas the elongation of pure gold wire
increases up to twenty times on comparable exposure.
It will be appreciated that these properties of the strengthened wire
render it very much more resistant to deformation and breakage than pure
gold wire on exposure to vibration, further thermal treatment and
mechanical shock, and thus render them especially suitable for use as
bond-wire in electrical circuits. It will be understood that low
resistance to deformation of bond-wire can lead to wire sag and tab
shorting, especially in ceramic encapsulated devices, and to moulding
failure in plastic encapsulated devices.
A particular feature of Au-Ti alloy material made by a method according to
the invention is that if it is heat treated to give desired elongation and
strength properties, heat treatment beyond the minimum time required to
give the minimum required elongation and strength properties can further
increase rather than reduce elongation and strength. By comparison, pure
gold material has its maximum strength in the as-drawn condition and heat
treatment, e.g. to increase elongation, will invariably reduce strength.
It will be understood that other materials made by a method in accordance
with the invention exhibit similar properties to the Au-Ti alloy material
described by way of example in respect of increasing, or at least
maintaining, their tensile strengths, and relatively small increase in
elongation compared with pure gold material, on exposure to temperatures
up to about 500.degree. C.
Table V below compares the hardness of two 250 .mu.m diameter wires made by
a method according to the invention of composition Au-1wt%Ti and Au-1wt%Zr
using various periods of heat treatment at 520.degree. C.
TABLE V
______________________________________
Time at Hardness (Hv)
500.degree. C. (hours)
Au-1 wt % Ti
Au-1 wt % Zr
______________________________________
0 62 63
2 170 174
4 162 171
6 158 169
8 156 166
10 154 160
100 130 146
1000 116 131
______________________________________
Materials made by a method in accordance with the invention exhibit the
ability to be satisfactorily bonded to members of aluminium, gold and
aluminium and gold based materials using conventional ultrasonic bonding
techniques.
Such bonds exhibit increased strength compared with bonds between pure gold
and aluminium. However for use in such techniques material made by a
method according to the invention is preferably first provided by
conventional methods with a thick coating of pure gold, silver, aluminium
or copper, or an alloy of these metals, or comprising layers of two or
more of these metals. The coating thickness may be required to be
comparable with or greater than the diameter of the gold alloy material in
order to obtain a satisfactory ball prior to bonding. A coating of such
thickness prevents the formation of oxides of the reactive metal component
of the gold alloy material that impair ball formation.
Materials made by a method according to the invention may also be provided
with a coating of a plastics material, if desired, without significantly
affecting their desirable properties.
It will be appreciated that the final heat treatment step of the method
according to the invention should be carried out before the application of
the above-described coatings.
Materials made by a method according to the invention by and large show
improved resistance to aggressive corrosive environments compared with
known gold, aluminium, gold based and aluminium based materials. For
example, Au-1wt%Ti has one tenth of the dissolution rate of pure gold in
aqua regia. This is of considerable importance in the case of bond-wires
in plastic encapsulation devices, i.e. bond-wires in electronic circuits
encapsulated in plastics material, since in such devices acidic vapours
are frequently produced in the region of bond-wires that can cause
corrosion fatigue in known bond-wires.
It will be understood that whilst in the case of material in the form of a
fine wire made by a method in accordance with the invention the
mechanically working step will comprise, at least in the latter stages, a
drawing process, in other methods in accordance with the invention the
mechanical working step may comprise rolling or other working processes.
A further feature of a method according to the invention is that fewer
processing stages are required to reduce an ingot of given size to wire,
sheet or strip material of desired dimensions than are required with gold
and gold based materials. This is due to the greater tensile strength of
the gold alloys used in methods according to the invention together with a
ductility at least comparable to gold and gold-based alloys. By way of
example of this feature it is found that the reduction of a 1 mm diameter
wire of pure gold to 25.mu.m diameter by wire drawing typically requires
about 100 stages whereas a Au-1wt%Ti wire of 1 mm diameter can be reduced
to 25.mu.m during a method according to the invention in less than 50
stages. This represents an elongation of length of more than 7% per stage.
It will be understood that material made by a method in accordance with
the invention may, if desired, include small amounts of additive other
than one or more of the metals titanium, lutetium, zirconium, scandium and
hafnium For example a wire made by a method in accordance with the
invention may for example include an amount less than 0.05 wt% or iron to
confer upon it thermoelectric characteristics so that it may be used as
thermocouple wire.
Top